Bandgap voltage reference circuits generate a reference voltage that is relatively stable over a wide temperature range by balancing a voltage having a negative temperature coefficient (TC) and which is thus complementary to absolute temperature (CTAT) with a voltage having a positive temperature coefficient (TC) and which is thus proportional to absolute temperature (PTAT). Typically, the forward-biased p-n junction of a diode or the forward-biased base-to-emitter junction of a transistor provides the CTAT voltage, and the thermal voltage of a diode or transistor provides the PTAT voltage. Generally, the two voltages are scaled or voltage-divided as necessary and summed to produce the temperature-stable reference voltage.
The above-described concept is schematically and graphically depicted in FIG. 1, wherein it is shown that the base-to-emitter voltage VBE of a transistor T, having a temperature coefficient (TC) of approximately negative 2 millivolts (mV) per degree Celsius, is summed with the thermal voltage Vt of a transistor which is scaled by factor K. The result is a reference voltage VREF that is equal to VBE plus the product of a scaling constant K and the thermal voltage Vt. Typically, VREF is from about 1.2 to 1.3 V depending on the particular technology of the components, and is close to the theoretical bandgap of Silicon at 0 K.
The continued trend toward producing ever smaller and more portable electronic devices requires that power consumption be reduced in order to increase battery life. In order to reduce power consumption, the supply, operating, and reference voltages supplied to and used by the circuitry within such devices must also be reduced. However, it is difficult to further reduce supply and reference voltages since typical bandgap voltage reference circuits provide a minimum reference voltage VREF of about 1.2 to 1.3 V and therefore require a supply voltage of at least approximately 1.4 V (one drain-source voltage drop higher than the reference voltage).
Some bandgap circuits that do provide reference voltages of less than 1.2V use an approach commonly referred to as fractional VBE, wherein a fraction of the CTAT voltage drop across a base-to-emitter p-n junction is derived, typically via voltage division. A scaled PTAT voltage which is derived from a PTAT current is added to the CTAT fractional VBE to thereby produce a voltage that is relatively stable across a wide temperature range. The bandgap voltage reference circuits that use the fractional VBE approach, however, require additional voltage-dividing circuitry, such as resistors, that undesirably consume relatively large amounts of real estate on integrated circuit chips and raise power consumption.
Therefore, what is needed in the art is a bandgap voltage reference circuit that produces a reference voltage of less than 1.2 Volts.
Furthermore, what is needed in the art is a bandgap voltage reference circuit that operates with a reduced minimum supply voltage and thereby consumes less energy.
Moreover, what is needed in the art is a bandgap voltage reference circuit that provides a reference voltage of less than 1.2 Volts without the disadvantages of the fractional VBE approach.